A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.
See:
*https://support.mozilla.org/kb/Server+not+found
*https://support.mozilla.org/kb/Firewalls
You can use one of these to start Firefox in <u>Safe mode</u> to see if that works:
*On Windows, hold down the Shift key while starting Firefox with a double-click on the Firefox desktop shortcut
*On Mac, hold down the Options key while starting Firefox
*Help > Restart with Add-ons Disabled
*https://support.mozilla.org/kb/Safe+Mode
See also:
*http://kb.mozillazine.org/Browser_will_not_start_up